spi: Use bus probing for SFDP

Only probe for the SFDP signature and split the actual SFDP parsing
into a chip preparation function.

Change-Id: I182d0a386bb2fd11951a1c9f2f965ce68ff57cf0
Signed-off-by: Nico Huber <nico.h@gmx.de>
Reviewed-on: https://review.sourcearcade.org/c/flashprog/+/427
diff --git a/spi.c b/spi.c
index d674a6d..7cacca3 100644
--- a/spi.c
+++ b/spi.c
@@ -170,6 +170,7 @@
 	{ 0, ID_SPI_RES3,	probe_spi_res,		NULL },
 	{ 0, ID_SPI_RES2,	probe_spi_res,		NULL },
 	{ 0, ID_SPI_RES1,	probe_spi_res,		NULL },
+	{ 0, ID_SPI_SFDP,	probe_spi_sfdp,		NULL },
 	{ 1, ID_SPI_ST95,	probe_spi_st95,		(void *)(uintptr_t)3 },
 	{ 1, ID_SPI_ST95,	probe_spi_st95,		(void *)(uintptr_t)2 },
 };